Generally, within a radio receiver, a phase detector is used to recover the phase information from the received signal. In a system utilizing a digital phase modulation scheme, a symbol slicer determines the symbol that is represented by the recovered phase information. Several methods of detecting the phase of a received signal are available.
One method of detecting the phase of a received signal is direct phase digitizing with an apparatus as illustrated in FIG. 1. Using this method, the direct phase digitizing apparatus accepts a voltage limited IF signal 101. A reference oscillator 103 generates a reference oscillator signal 107. The reference oscillator signal 107 drives a phase sector counter and docks a zero-crossing detector 105. The phase sector counter generates an estimated phase map 109. The zero-crossing detector 105 indicates when the limited IF input signal 101 crosses a predetermined voltage level. The estimated phase map is then sampled at each occurrence of a zero-crossing of the limited IF input signal 101 using an N-bit latch 113, forming a digital phase signal 111.
Using this method, the resolution of the digital phase signal 111 is directly dependent upon the frequency of the reference oscillator according to the following equation: f.sub.ref =2.sup.N f.sub.i, where N is the number of bits desired in the phase word, f.sub.i is the input frequency of the limited IF signal and f.sub.ref is the frequency of the reference oscillator. A system having an intermediate frequency of 1.25 MHz would require a reference frequency of 40 MHz to produce a 5-bit phase word and a resolution of 32 phase sectors. A direct phase digitizer which could produce a phase word having the same resolution and using a lower frequency reference oscillator would be advantageous. A lower frequency reference oscillator reduces the size, cost and power consumption required by the phase demodulator. Size, cost and power consumption are important criteria used to evaluate the performance of a radio receiver.